Sleep apnea/hypopnea affects around 5% of the adult U.S. population. Its short-term effects consist of complete (apnea) or partial (hypopnea) termination of airflow, decreased oxygen in the blood, increased CO2 in the blood, interrupted sleep, and excessive daytime sleepiness. Long-term effects may include hypertension, diabetes, heart attack, stroke, arrhythmia and brain damage.
The principal forms of sleep apnea are: 1) obstructive sleep apnea (OSA), characterized by a physical blockage of the upper airway during sleep, 2) central sleep apnea (CSA), caused by a decreased central respiratory drive during sleep, and 3) mixed sleep apnea, which includes components of both OSA and CSA. OSA is the most common and dangerous of all sleep-related breathing disorders. While CSA is uncommon in its pure form, it is prevalent in patients with congestive heart failure, as a component of Cheyne-Stokes respiration.
The obstructive component in OSA is related to decreased tone in the upper airway as the muscles relax during sleep. During normal respiration, upper airway patency is maintained by the negative pressure reflex, which activates pharyngeal dilators in response to negative transthoracic pressure during inspiration. In apneic patients, the negative pressure reflex is insufficient to maintain patency during sleep. Here, the negative pressure created during inspiration is sufficient to constrict or collapse the lumen of the flaccid airway.
The treatment of choice for sleep apnea is continuous positive air pressure (CPAP). Basically, CPAP maintains an open airway by inflating it with pressurized air through a nose or face mask. Used properly, CPAP is 100% effective for treating OSA. Although CSA has a neurological origin, it has also been successfully treated with positive air pressure. Despite its efficacy, however, CPAP treatment is poorly tolerated by sleep apnea patients. In one recent survey, CPAP non-compliance (less than 4 h/night) was reported in between 46% and 83% of patients [1]. Reasons for non-compliance include discomfort, claustrophobia, pressure sores, dry nose or mouth, and machine noise.
The most common alternative to CPAP is a surgical removal of the uvula, caudal soft palate, and tonsils. This procedure has a success rate of about 50%. Other surgical treatments, such as tongue reduction, advancement of the tongue, tracheostomy, or implants to stiffen the soft palate have limited benefit relative to their invasiveness, risk, and irreversibility. Non-surgical approaches such as weight loss, medication, changes in sleeping position or dental appliances also suffer from limited effectiveness or compliance.
Implantable medical devices are currently under investigation as a method to detect and/or treat sleep apnea. Such devices are similar in their general design to cardiac pacemakers and share in many of the advantages of this mature technology.
With regard to detection, implantable devices have been described that detect apnea by monitoring the bioelectric activity of the diaphragm, intercostal muscles, or their efferent nerves. Other devices monitor the bioelectric activity of upper airway muscles or their efferent nerves. Still others monitor implanted sensors for indications of, for example, thoracic pressure or blood oxygenation.
With regard to treatment, implantable devices have been described that terminate apnea using drug delivery, atrial overdrive pacing or electrical stimulation of the nerves or muscles that control respiratory activities. For OSA, electrical stimulation has been described that maintains patency by activating upper airway muscles or the efferent nerves controlling them. For CSA, treatments that elicit breathing by electrically stimulating the diaphragm, intercostal muscles, or their efferent nerves have been described.